NextJS contact form with nodemailer throws Error 500 in Production

I have a simple contact form in a NextJS app and I wanted to switch from emailjs to something custom.

The contact form works perfect in development, but in production(cpanel, not vercel) I get a status 500 error(Internal Server Error). I've researched around and implemented async/await, promises, res.send status and everything I've found but I still can't find a way through.

Here's the post method:

    const checkForm = async () => {
        try {
            const values = await form.validateFields();

            fetch('/api/contact', {
                method: 'POST',
                headers: {
                    Accept: 'application/json, text/plain, */*',
                    'Content-Type': 'application/json'
                },
                body: JSON.stringify(values)
            });
        } catch (error) {
            console.log('Failed:', error);
        }
    };

And the /api/contact

import nodemailer from 'nodemailer';

export default async (req, res) => {
    const { first_name, last_name, email, phone_number, subject, message } = req.body;
    const transporter = nodemailer.createTransport({
        service: 'gmail',
        auth: {
            user: process.env.EMAIL,
            pass: process.env.PASSWORD
        }
    });

    await new Promise((resolve, reject) => {
        transporter.verify(function (error, success) {
            if (error) {
                console.log(error);
                reject(error);
            } else {
                console.log('Server is ready to take our messages');
                resolve(success);
            }
        });
    });

    const mailOption = {
        from: `${email}`,
        replyTo: `${email}`,
        to: `${process.env.EMAIL}`,
        subject: `${subject}`,
        html: `<p><strong>Name:</strong> ${first_name + ' ' + last_name}</p>
        <p><strong>Email:</strong> <a href="mailto:${email}">${email}</a></p>
        <p><strong>Phone number:</strong> ${phone_number}</p>
        <p><strong>Message:</strong><br /> ${message}</p>
      `
    };

    await new Promise((resolve, reject) => {
        transporter.sendMail(mailOption, (err, info) => {
            if (err) {
                console.error(err);
                reject(err);
                res.send('error:' + JSON.stringify(err));
            } else {
                console.log(info);
                resolve(info);
                res.send('success');
            }
        });
    });

    res.status(200).json({ status: 'OK' });
};